FIG. 5 of the attached drawings shows a conventional inverter apparatus. The reference numeral 1 denotes a commercial power source; 2, a power rectifier for converting the output of the commercial power source 1 to direct current; 3, a capacitor for filtering current; 4, an inverter for converting the direct current to alternating current of optional frequency and voltage; 5, a motor that is a load; 6 and 7, current detectors by which conversion is made to a voltage signal with or at a predetermined ratio in accordance with the magnitude of the output current (including positive and negative polarities); 8, a direct-current bus voltage detector in which the magnitude of direct-current bus voltage is converted to a voltage signal with a lower predetermined value; 9, a setter for setting a desired speed for the motor 5; and 10 and 11, start switches for setting a rotational direction of the motor 5, respectively.
Further, the reference numerals 13 and 14 denote pull-up resistors in which, even at the OFF position of the start switches 10 and 11, a signal to a computer 16 does not become unselective; 16, a computer (for example, a microprocessor) for synthetically discriminating or judging various signals from the setter 9, the start switches 10 and 11, the current detectors 6 and 7 and the direct-current bus voltage detector 8 to send out ON/OFF signals to switching element of the inverter 4; and 17, an amplifier circuit for electrically insulating and amplifying the ON/OFF signals outputted from the computer 16 by light and the like and, subsequently, for sending out signals to various elements of the inverter 4.
In this connection, in FIG. 5, the reference numeral 101 denotes an element which is used in the power rectifier 2, and generally is a diode; 102, an element which is used in the inverter 4, and which may be a transistor, a gate turn-off thyristor (GPO), or an insulating-gate bipolar transistor (IGBT); and 103, a diode connected to the transistor 102 in reversed polarity.
Operation will next be described. In a circuit illustrated in FIG. 5, two methods have conventionally been executed. Of the two methods, a first control system will be described with respect to a flow chart illustrated in FIG. 6, while a second control system will be described with respect to a flow chart illustrated in FIG. 7.